In mammals, the hemodynamics of venous return (i.e. bloodflow through the veins of the body) are influenced by three significant factors, including: (1) the action of the heart itself in continually pumping blood through the arterial circulation and into the capillary beds of the vascular tree; (2) a "muscle pump" effect created by movements of the visceral organs and somatic muscles; and (3) a "respiratory pump" effect created by the normal cyclic variations in intrathoracic pressure which occur during respiration due to respiratory movement of the thoracic muscles and diaphragm.
In many mammals, small semilunar valves known as "venous valves" (syn. valvulae vienosa) are found within the extremity veins. Such venous valves function as one-way check valves to maintain the flow of venous return blood in the direction toward the heart, while preventing blood from backflowing in a direction away from the heart. Such venous valves are particularly important in the veins of the lower extremities, as venous blood returning from the lower extremities is required to move against a long hydrostatic column, especially when the subject animal is in a standing or upright position.
Acquired venous valvular incompetence occurs in humans as a result of trauma (e.g. crushing injury to the vein) or disease (e.g. inflammatory thrombophlebitis). Hereditary factors may also result in incompetence or absence of venous valves in some individuals.
Any condition which results in chronic incompetence or absence of venous valves, especially in the veins of the lower extremities, is likely to give rise to troublesome pathologic consequences. For example, incompetence or absence of venous valves at the saphenofemoral or saphenopopliteal junction may result in varices of the primary (and/or secondary) veins of the lower leg and ankle. Such veins tend to become enlarged, dilated and tortuous, often resulting in pain as well as noncosmetic appearance. Additionally, incompetence or absence of venous valves may cause deep venous hypertension of the lower limbs with resultant lymphedema, aberrant pigmentation of the skin and, in severe cases, the formation of necrotizing lesions (i.e. "venous ulcers"). If not successfully treated, the presence of necrotizing lesions may eventually require limb amputation.
Several surgical procedures have been developed for treating venous valvular insufficiency. Such surgical procedures include: (a) surgical valvuloplasty procedures wherein some or all of the incompetent valves are surgically repaired or reconstructed; see, Garcia-Rinaldi, R, et al., "Femoral Vein Valve Incompetence: Treatment with a Xenograft Monocusp Patch", Journal of Vascular Surgery, Vol. 3, No. 6; pp. 932-935 (June 1986); (b) surgical bypass procedures wherein a segment of viable vein is utilized to bypass a region of veins having incompetent or absent venous valves positioned therein; and (c) surgical valve transplantation procedures whereby incompetent or absent venous valves are excised, removed and replaced by transplanted segments of viable vein having functioning venous valves therein. See, Gotlob R. and May R. Venous valves V Section 3, pp. 183-208, "Reconstruction of Venous Valves" (Springer-Verlang/Wien 1986). Additionally, efforts have been directed toward the development of man-made prosthetic venous valves for surgical implantation in patients who suffer from venous valvular incompetence. See, Hill, R. et al., "Development of a Prosthetic Venous Valve; Journal of Biomedical Materials Research, Vol. 19, pp. 827-832 (1985); Taheri, S. A. et al., "Experimental Prosthetic Vein Valve", The American Journal of Surgery, Vol. 156, pp. 111-114 (1988).
Although surgical venous valve "bypass" and "transplant" procedures may be viable modes of treating chronic venous valvular insufficiency, the process of locating and harvesting autograft tissue (i.e. segments of viable vein taken from the patient's own veins and used for subsequent transplant or bypass graft) are often problematic (a) because of difficulties encountered in locating suitable segments of vein having viable venous valves therein and/or (b) because of the necessity for performing a separate incision or second surgery to harvest a venous valve autograft from a separate location and/or (c) size mis-matching of the harvested venous valve autograft relative to the implant site, as may result in subsequent failure of the implanted valve.
In view of the limitations and shortcomings in the prior art uses of autograft tissue for venous valvular transplantation or bypass grafting, it is desirable to develop methods of preparing and preserving venous valvular allograft or heterograft tissues harvested from the veins of animals or cadaverous human sources for subsequent surgical implantation (i.e. valvular transplantation or bypass grafting) in humans suffering from venous valvular insufficiency. Also, it is desirable to design and develop implantable artificial venous valves (i.e. made of synthetic materials) capable of carrying out the same physiological functions as venous valves of natural origin.
In furtherance of programs aimed at developing means and methods of preparing venous valvular implants of biological or artificial origin, there exists a need for a mechanical device capable of testing the function of such natural or artificial venous valves. Such venous valve testing device will preferably be capable of mimicking or simulating the normal physiological and hemodynamic conditions encountered by venous valves in situ. The development of such venous valve function testing device will provide a means for conveniently testing new methods and materials for preparing artificial and/or natural venous valves for subsequent surgical implantation and will minimize the need for conducting such testing and experimentation in live animals.
While the prior art has included several devices for functional testing of heart valves (e.g. mitral valves, aortic valves). Such heart valve function testing devices are not suitable for use in testing the function of venous valves because the hemodynamic variables encountered by venous valves in situ are grossly different from those encountered by heart valves. One example of a commercially available heart valve function testing device of the prior art is known as the "Heart and Load" Model MHL 6991, (Vivitro Systems, Inc., Victoria, British Columbia).
It is noted that, in the prior art, there has also existed at least one durability/fatigue testing device for testing the long term (e.g. 5 months) operability of venous valves. However, such durability/fatigue testing device serves only to effect continual oscillatory flow of liquid through the subject valve and does not incorporate means for altering and changing hemodynamic pressure and flow variables (i.e. "respiratory pump" effects, capacitance changes, etc. ) to permit simulation of various hemodynamic pressure and/or flow waveforms as may be encountered by venous valves in situ. One example of a venous valve durability/fatigue testing device is disclosed in Taheri, S. A. et al., "Experimental Prosthetic Vein Valve", American Journal of Surgery, Vol 156, pp. 111-114 (1988).
Given the foregoing shortcomings of the prior art, there exists a need for a mechanical venous valve function testing device which is capable of mimicking or simulating various hemodynamic and flow conditions encountered by venous valves in situ.